9 November 2021

Current Insights Into Respiratory Virus Transmission and Potential Implications for Infection Control Programs: A Narrative Review

Publication: Annals of Internal Medicine
Volume 174, Number 12


Policies to prevent respiratory virus transmission in health care settings have traditionally divided organisms into Droplet versus Airborne categories. Droplet organisms (for example, influenza) are said to be transmitted via large respiratory secretions that rapidly fall to the ground within 1 to 2 meters and are adequately blocked by surgical masks. Airborne pathogens (for example, measles), by contrast, are transmitted by aerosols that are small enough and light enough to carry beyond 2 meters and to penetrate the gaps between masks and faces; health care workers are advised to wear N95 respirators and to place these patients in negative-pressure rooms. Respirators and negative-pressure rooms are also recommended when caring for patients with influenza or SARS-CoV-2 who are undergoing “aerosol-generating procedures,” such as intubation. An increasing body of evidence, however, questions this framework. People routinely emit respiratory particles in a range of sizes, but most are aerosols, and most procedures do not generate meaningfully more aerosols than ordinary breathing, and far fewer than coughing, exercise, or labored breathing. Most transmission nonetheless occurs at close range because virus-laden aerosols are most concentrated at the source; they then diffuse and dilute with distance, making long-distance transmission rare in well-ventilated spaces. The primary risk factors for nosocomial transmission are community incidence rates, viral load, symptoms, proximity, duration of exposure, and poor ventilation. Failure to appreciate these factors may lead to underappreciation of some risks (for example, overestimation of the protection provided by medical masks, insufficient attention to ventilation) or misallocation of limited resources (for example, reserving N95 respirators and negative-pressure rooms only for aerosol-generating procedures or requiring negative-pressure rooms for all patients with SARS-CoV-2 infection regardless of stage of illness). Enhanced understanding of the factors governing respiratory pathogen transmission may inform the development of more effective policies to prevent nosocomial transmission of respiratory pathogens.

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Information & Authors


Published In

cover image Annals of Internal Medicine
Annals of Internal Medicine
Volume 174Number 12December 2021
Pages: 1710 - 1718


Published online: 9 November 2021
Published in issue: December 2021




Michael Klompas, MD, MPH
Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, and Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts (M.K., C.R., M.A.B.)
Donald K. Milton, MD, DrPH
Maryland Institute for Applied Environmental Health, School of Public Health, University of Maryland, College Park, Maryland (D.K.M.)
Chanu Rhee, MD, MPH
Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, and Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts (M.K., C.R., M.A.B.)
Meghan A. Baker, MD, ScD
Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, and Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts (M.K., C.R., M.A.B.)
Surbhi Leekha, MBBS, MPH
Department of Epidemiology and Public Health, University of Maryland School of Medicine, Baltimore, Maryland (S.L.).
Grant Support: By grant 6U54CK000484-04-02 from the Centers for Disease Control and Prevention (Drs. Klompas, Rhee, and Baker); grants from the National Institute of Allergy and Infectious Diseases Centers of Excellence for Influenza Research and Surveillance (HHSN272201400008C), the Centers of Excellence for Influenza Research and Response (75N93021C00014), the Centers for Disease Control and Prevention (200-2020-09528), and the National Science Foundation (2034755) (Dr. Milton); and grant 6 U01CK000556-02-02 from the Centers for Disease Control and Prevention (Dr. Leekha).
Corresponding Author: Michael Klompas, MD, MPH, Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, 401 Park Drive, Suite 401 East, Boston, MA 02215; e-mail, [email protected].
Author Contributions: Conception and design: M. Klompas, C. Rhee, M.A. Baker, S. Leekha.
Analysis and interpretation of the data: S. Leekha.
Drafting of the article: M. Klompas, S. Leekha.
Critical revision of the article for important intellectual content: M. Klompas, D.K. Milton, C. Rhee, M.A. Baker, S. Leekha.
Final approval of the article: M. Klompas, D.K. Milton, C. Rhee, M.A. Baker, S. Leekha.
This article was published at on 9 November 2021.

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Michael Klompas, Donald K. Milton, Chanu Rhee, et al. Current Insights Into Respiratory Virus Transmission and Potential Implications for Infection Control Programs: A Narrative Review. Ann Intern Med.2021;174:1710-1718. [Epub 9 November 2021]. doi:10.7326/M21-2780

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